1. Field of the Invention
The present invention relates generally to vibration damping devices suitably used as an automobile engine mount, and the like, for example, and more particularly to an engine mount of novel construction, which is capable of advantageously exhibiting vibration damping effect with respect to various kinds of vibration, on the basis of flow action of the fluid sealed therein.
2. Description of the Related Art
A fluid-filled vibration damping device is known as one type of vibration dam ping device installed between two members constituting a vibration transmitting system, like an engine mount interposed between a body and a power unit of a automotive vehicle. Typically, a fluid-filled vibration damping device includes a pressure receiving chamber and an equilibrium chamber, which are both filled with non-compressible fluid, and an orifice passage for permitting a fluid communication between the pressure receiving chamber and the equilibrium chamber. Thus, the fluid-filled vibration damping device is able to exert vibration damping effect on the basis of flow action of the non-compressible fluid forced to flow through the orifice passage between two chambers during input of vibration.
There is known with respect to such a fluid-filled vibration damping device that the vibration damping effect on the basis of flow action of the non-compressible fluid is effective only to a relatively narrower vibration frequency range, which is previously tuned by suitably adjusting the length and the cross sectional area of the orifice passage.
Meanwhile, many vibration damping devices are required to exhibit vibration damping effect with respect to input vibration of a plurality of frequency ranges or alternatively over a wide frequency range. One proposed major to meet this end is disclosed in JP-B-2516487 wherein the fluid filled vibration damping device further includes a rubber elastic plate disposed in a partition wall partitioning the pressure receiving chamber and the equilibrium chamber from each other. With this arrangement, when subjected to input of vibration of frequency higher than the tuning frequency of the orifice passage, the vibration damping device is able to preventing increase in its dynamic spring constant by transmitting fluid pressure fluctuation in the pressure receiving chamber to the equilibrium chamber through elastic deformation of the rubber elastic plate, thereby providing enhanced vibration damping performance with respect to higher frequency vibration.
In the conventional fluid-filled vibration damping device of construction as disclosed in JP-B-2516487, while somewhat improvements are admitted in terms of vibration damping performance at a higher frequency range that is higher than the tuning frequency range of the orifice passage, the vibration damping performance exhibited by positively utilizing flow action of the fluid flowing through the orifice passage is only limited to low frequency and large amplitude vibration. Therefore, the conventional fluid-filled vibration damping device still ensures its advantageous damping effect only at a narrow frequency range, and is not able to exhibit satisfactory damping effect. For instance, while an engine mount for automotive vehicle is required to exhibit excellent vibration damping performance not only for vibration excited during running conditions, e.g., low frequency and large amplitude vibration, such as engine shakes, and high frequency small amplitude vibration such as booming noises, but for vibration excited during idling conditions, e.g., medium frequency and medium amplitude vibration corresponding to engine idling vibration, it is still difficult for the conventional fluid-filled vibration damping device to sufficiently achieve the required vibration damping performance.